The present invention is directed to fluid catalytic cracking catalysts comprising zeolite particles which are coated with bayerite alumina, and to FCC processes which utilize the subject catalyst.
Catalytic cracking is a petroleum refining process which is applied commercially on a very large scale. A majority of the refinery gasoline blending pool in the United States is produced using the fluid catalytic cracking (FCC) process. In the catalytic cracking process heavy hydrocarbon fractions are converted into lighter products by reactions taking place at elevated temperature in the presence of a catalyst, with the majority of the conversion or cracking occurring in the vapor phase. The feedstock is thereby converted into gasoline, distillate and other liquid cracking products as well as lighter gaseous cracking products of four or less carbon atoms per molecule. The gas partly consists of olefins and partly of saturated hydrocarbons.
Cracking catalysts used in FCC processes are fine porous powders composed of oxides of silica and aluminum. Other elements may be present in very small amounts. Either Bronsted or Lewis acid sites associated with the aluminum are believed to initiate and accelerate carbocation reactions that cause molecular size reduction of the petroleum oils under the FCC reactor conditions. When aerated with gas, the powder attains a fluid-like state that permits its circulation through the various FCC process zones.
During the cracking reactions some heavy material, known as coke, is deposited onto the catalyst. This reduces the activity of the catalyst. After removal of occluded hydrocarbons from spent cracking catalyst, regeneration is accomplished by burning off the coke to restore catalyst activity. The three characteristic process zones of the FCC process are composed of: a cracking step in which the hydrocarbons are converted into lighter products, a stripping step to remove hydrocarbons adsorbed on the catalyst and a regeneration step to burn off coke from the catalyst. The regenerated catalyst is then reused in the cracking step.
Various attempts have been made to improve the performance of FCC catalysts. These catalysts have been formed from mixtures of zeolites with an active matrix material, such as various forms of alumina, or have been coated. For example, JP laid-open application SHO 58-112,051 discloses the formation of a zeolite, which has been coated with a metallic oxide prior to incorporation into the catalyst composition. The zeolite is dispersed in an aqueous acidic solution of the metal salt and then treated with ammonia water to raise the pH to about 9 causing the metal to deposit as the hydroxide on the surface of the zeolite. The resultant coating is a relatively amorphous alumina.
In U.S. Pat. No. 4,332,699, a pseudo-boehmite alumina was coated on the surface of zeolite particles via a low pH process. The crystallinity of zeolite has been deemed susceptible to damage by subjecting it to very high pH conditions. Thus, precipitation processes have been done under controlled pH values of 7-9 more normally 7 to 8. Under these conditions the alumina coatings are of boehmite or pseudo-boehmite structure.
FCC catalysts have also been formed from zeolites which are augmented by active matrix materials of aluminas. For example, U.S. Pat. No. 5,168,086 discloses the mixing of bayerite/eta alumina particles into the cracking catalyst matrix to improve its tolerance to nickel-containing feedstocks. The zeolite is mixed with the alumina along with other conventional matrix components and then calcined to form the catalyst particles.
In cracking there is a desire to optimize output. The scale of cracking is such that even what appears to be a modest improvement may have a large effect on a refinery""s profitability. There has been a desire to tailor catalysts to achieve specific refinery objectives (e.g., maximizing output of certain types of molecules). For example, refiners often desire to increase or maximize their output of light cycle oil (LCO). They also have the desire to minimize the amount of uncracked xe2x80x9cbottomsxe2x80x9d, especially where the feedstock is heavy feed, such as resids. While refineries wish to achieve these various goals, they also want to avoid/minimize the output of coke and hydrogen from the FCC process.
Cracking catalysts must be able to crack the range of constituents in a feedstock to achieve the desired output. In this context, the cracking catalyst itself may contain various components ranging from zeolites, active matrix materials (e.g., alumina, relatively inactive matrix materials (e.g., clay) to binders (e.g., sols). Cracking catalysts, especially for FCC processes are necessarily constrained as to their particle size by virtue of the fact that the catalyst particles must be adequately fluidizable in the process. An additional constraint is that the catalyst must be attrition resistant. The requirement for attribution resistance generally means that a significant amount of clay and binder must be present in the catalyst particle. Thus, there is only limited room in the catalyst particle for those components that are responsible for the majority of the cracking function (i.e., zeolite/active matrix).
While modern cracking catalysts have made significant strides to improve catalytic performance, there still is the need to provide catalysts which can exhibit improved cracking of heavy bottom materials or resids, without increasing the alumina content of the catalyst. Further, there is the need to provide a FCC catalyst which minimizes coke/hydrogen formation at a given bottoms cracking performance level.